1. Field of the Invention
Implementations of various techniques described herein generally relate to techniques for processing seismic data, and more particularly, to techniques for estimating elastic parameters.
2. Description of the Related Art
The following descriptions and examples do not constitute an admission as prior art by virtue of their inclusion within this section.
When a material is said to be anisotropic it means that its properties vary with the direction in which it is measured. For example, the speeds of compressional waves in a sedimentary rock are typically lower when measured in the vertical direction compared with a similar measurement taken in a horizontal direction.
Typically, anisotropic formations result from fine layering, which tends to be horizontal, resulting in a so-called Vertical Transverse Isotropic (VTI) composite. It is also possible for these layers to contain aligned, near-vertical fractures, resulting in a composite rock that may possess orthorhombic symmetry.
Seismic measurements, such as seismic reflection surveys, are sensitive to the elastic properties of the rock, and thus, the elastic anisotropy. Failure to account for elastic anisotropy can lead to poor seismic images that may be poorly focused. Poor seismic images may also represent inaccuracies in depth that may lead to expensive failures in hydrocarbon field development. For these reasons, measurements of seismic anisotropy are of great value in the seismic industry.
Seismic anisotropy can be estimated using appropriately designed Vertical Seismic Profile (VSP) surveys. If anisotropy estimates are to be determined over an interval intersected by a well-bore, then placement of a receiver array over this depth range may allow the extraction of elastic anisotropy parameters using several different methods.
One particularly advantageous VSP arrangement is the walkaway VSP. In this configuration the source is moved to progressively further offsets along the surface, while the receivers remain in a fixed location. The walkaway VSP may provide a 2-dimensional (2D) image of the subsurface that can be of higher resolution than surface seismic data. Additionally, the walkaway VSP may provide more continuous coverage than other VSP types, such as an offset VSP. The receivers are typically 3-component and are sensitive to motion along three orthogonal directions.
In a 3-dimensional (3D) VSP survey, seismic sources are deployed over an area forming a grid or spiral pattern. Such surveys may be used to enhance 3D seismic images in areas where the surface seismic data do not provide an adequate image. The inadequacy of the image may be due to near-surface effects, or surface obstructions.
Typically, a 3-component wavefield may be decomposed into polarization and slowness components that may be resolved along the well-bore. The polarization and slowness components may then be inverted to yield estimates of the elasticity. These wavefield properties of polarization and slowness components also provide an advantage in that they are dependent only on the local elasticity over the receiver array. As such, the VSP surveys may be used in the formations with arbitrary overburden complexity.
In the case that the geology can be considered plane layered, it may be possible to construct slowness curve data from measurements of the direct P-wave arrival using a walkaway VSP.
In the case that 3D VSP data are available, a slowness surface may be constructed. Such 3D data may allow the inversion of more complicated forms of anisotropy that describe propagation effects that change with the azimuth angle as well as changes with respect to the inclination angle.